1. Field of the Invention
The invention relates to scanning tunneling microscopes (STM), and more particularly to an easily removable STM tip treatment attachment for a scanning tunneling microscope that readily converts the STM to a spin polarization STM (SP-STM).
2. Description of the Prior Art
The scanning tunneling microscope (STM) has revolutionized the field of surface science, and a new field of nanotechnology has developed in which the STM is used as a primary tool for nanofabrication and characterization of nanoscale materials and structures. In a STM system, an atomically sharp wire tip is positioned by piezoelectric actuators above the surface of an electrically conductive sample. When the tip/sample distance is sufficiently small, typically 5-15 angstroms, the application of a small voltage between the tip and the sample leads to a quantum mechanical tunneling current. This tunneling current decays exponentially with increasing tip/sample distance by about one order of magnitude per angstrom. Accordingly, the current is localized to the apex of the tip where tip and sample are the closest. In one use of a STM, if the tunneling current is maintained constant while scanning the tip above the sample surface, the surface topography can be imaged with atomic resolution.
STM users have long wanted to detect the local magnetization of the surface below the apex of a STM tip. This would be done by making the tip sensitive to the spin polarization of the tunneling electrons, thereby imaging the magnetic domain structure of the sample with ultimate resolution down to the atomic scale. In order to realize a spin-polarization STM (SP-STM), the STM tip must be made of a magnetic material that exhibits an intrinsic spin polarization close to the Fermi level.
Substantial progress toward spin-polarization STM has been made over the last fifteen years (Ref. 1). However, it is still carried out with difficulty. Only a very few groups have succeeded in adding a spin polarization function to their STM systems at all. For those groups that have succeeded, the main problem continues to be that major modifications to the existing STM system are required, and as a result, the system becomes more limited, almost wholly dedicated to SP-STM alone. This means a consequent lack of versatility and compatibility for other needed tasks in STM systems that have been modified for spin-polarization STM.
An ideal wire tip for SP-STM must possess a good signal-to-noise ratio that can only be achieved if the apex atom exhibits a high spin polarization. Since the presence of adsorbents typically reduces the spin polarization, a clean environment and an inert tip material must generally be maintained. The most widely used STM tip material is tungsten wire. However, tungsten wire requires an extremely high annealing temperature (>2000° C.) to clean. In one widely used STM tip carrier design, by Omicron Nanotechnology Corporation, the tungsten wire is mounted in a tip carrier that incorporates organic insulators and gold coatings which cannot stand such a high temperature.
In order to successfully flash a tungsten tip, a large temperature gradient (>1800° C.) is impressed across the millimeter-long tungsten wire. Dipolar interaction (Ref 1) between the magnetic tip and the sample due to the stray magnetic field should be as low as possible because it may modify or destroy the sample's intrinsic domain structure that is the subject of the investigation. To fulfill this requirement, the magnetic coating area must be localized to the sharp tip end (apex) of the tungsten wire.
The present invention is an easily removable STM tip treatment attachment for a scanning tunneling microscope that readily converts the STM to a spin polarization STM (SP-STM). The STM tip treatment device can not only provides the spin polarization function, but is capable of providing other functions as well, including flashing an STM tip to high temperature (>2000° C.) in an extremely localized fashion, general tip cleaning, and coating the tip apex with monolayers of coating materials.